New Insights into the Cleaning of Paintings

The Chemistry of Egg Binding Medium and Its
Interactions with Organic Solvents and Water
Antonella Casoli, Michela Berzioli, and Paolo Cremonesi
Abstract. The aim of this study was to gain a deeper insight into the properties of egg, used
as a binding medium, and its interactions with water and organic solvents. The research focused
on egg tempera films prepared in July 2007 and on a tempera layer prepared by researchers of the
Smithsonian’s Museum Conservation Institute in 1995. It also included a sixteenth- century panel
painting from the Pinacoteca di Siena. Two paint samples from this painting were analyzed by gas
chromatography coupled with mass spectrometry and identified egg as the binding medium. Ethanol
or acetone, isooctane, and water were used to test for leaching. In general, the main leaching occurs
from the fresher samples: using solvents of different polarity (from isooctane to water), the lipid
components are the most removed. It also was observed that pigmented layers are less affected by
leaching phenomena than a layer without pigments, in particular for lipid components. Finally, it
was noticed that unsaturated fatty acids were extracted mainly from fresh samples, with less from
the 1995 tempera and nothing from the sixteenth- century painting. However, the study carried out
on this painting showed that leaching is more pronounced for the lipid components, and amino acids
were also detected. These results were unexpected because mild cleaning tests were believed not to
affect a 500- year- old painting. These results might be regarded as guidelines to take into account for
cleaning paintings.
INTRODUCTION
Antonella Casoli and Michela Berzioli, Dipartimento
di Chimica, Università degli Studi di Parma,
viale G.P. Usberti, 17/a, 43124 Parma, Italy.
Paolo Cremonesi, Cesmar7–The Centre for the
Study of Materials for Restoration, Via Lombardia
41/43, 35020 Saonara, Italy. Correspondence:
Antonella Casoli, antonella.casoli@unipr.it;
Michela Berzioli, michela.berzioli@libero.it;
Paolo Cremonesi, paolocremonesi57@gmail
.com. Manuscript received 19 November 2010;
accepted 24 August 2012.
Egg tempera techniques employ the whole egg, or egg yolk and egg white separately,
for binding media purposes, in which pigments are dispersed, sometimes combined with
other materials, i.e., fig latex and cherry gum, depending on the painter’s requirements.
Egg tempera was traditionally used in the past, especially in the fourteenth and fifteenth
centuries in Italian painting.
Because detailed studies are available on this topic (Thompson, 1936; Boon et al.,
1997; Phenix, 1997), only a general description will be provided. Egg binding medium
is made up of proteins, lipids, polysaccharides, and inorganic compounds. Egg white is
a diluted water solution of proteins (mainly ovalbumin) and a small fraction of polysaccharides.
Once applied, this material becomes insoluble.
Egg yolk contains mostly lipids (66% in terms of mass) and proteins, as well as small
amounts of polysaccharides and inorganic compounds. Lipids are present in the yolk as
triglycerides (neutral lipids), phospholipids, and cholesterol. Triglycerides, the main lipids,
are the same type of compound as drying oils. However, in egg these lipids contain less unsaturated
fatty acids compared to oils. The fatty acid distribution in yolk lipids is saturated
38%, monounsaturated 42%, and polyunsaturated 20%. Their drying properties are not
as strong as oils, but they are subject to the same oxidative polymerization reactions that